Rushton’s theory has to explain away too many historical examples where intelligent human populations that normally invest significantly in their offspring also had high fertility for his take on the idea to be salvaged.
Examples that immediately come to mind are:
- Puritan New England had a birth rate in excess of 8, and it remains one of the highest ever for which there are good records to base estimates upon
- The birth rate of 19th century Britain was more than 4
- Early 20th century Russian women had more than 5 children (this high fertility rate was one of the strategic factors which motivated Wilhelmine Germany, which had a fertility rate of ~3, to start WWI: The German military command wanted to achieve dominance before Imperial Russia was in a position to demographically threaten Germany as the preeminent state of Continental Europe)
- Pre-WWII Japan averaged over 4 children per woman
- The average in Communist China before the one child policy was greater than 5
There are also intra-racial variances in births for which the theory has little or no explanation.
Black Americans have much lower fertility than black Africans; the birth rate of France after the Napoleonic Wars and throughout the 19th century was stagnant compared to either Britain or Germany; Scandinavian nations, even after immigrants are factored out, have higher birth rates than Germans.
The historical trend prior to the 20th century has been for European fertility to be comparable to low-IQ races. It is only by focusing on recent decades that Rushton’s theory superficially appears somewhat applicable. But because this time range is too narrow for fixed evolutionary factors to be a tenable explanation and because it ignores too many counter-examples outside of that time range, his r/K selection theory should be discarded.